Many slide parts such as drills or other tools, molds, and engine parts are industrially used. For example, in press mold working, the lubrication mechanism on the friction interface during processing greatly affects the quality characteristics such as reduction of burrs of a press molded product and the durability of the mold of a punch or die. This lubrication mechanism includes an injection mechanism, which injects a lubricant into the friction interface, and a friction reduction mechanism, which uses a lubricant to reduce friction on the friction interface.
One injection mechanism uses projections and depressions on the material surface to inject the lubricant. There are various sizes of projections and depressions (surface roughness) on the surface of a workpiece or mold. The lubricant applied to the workpiece or mold before processing is stored in the depressions on the surface (storage effect). If there is no space to which the lubricant escapes when the mold and the workpiece make contact each other, the lubricant is encapsulated in the depressions and the lubricant is supplied to the friction interface with this state maintained. The depressions containing the lubricant are referred to as micro pools or lubricant pools in plastic working. This type of injection mechanism is expected in all types of plastic working.
When the surface roughness of the workpiece or mold is increased in advance by acid corrosion, shot peening, or sand blast in order to take advantage of this storage effect, the lubricant is easily injected, friction is reduced by the friction reduction mechanism, and galling or welding can be prevented. With the progress of pressure working, a high hydrostatic fluid pressure applies to the lubricant in micro pools and supports part of the pressure of the processed surface. In this state, the contact ratio between the workpiece and the mold is prevented from increasing (friction reduction mechanism). The technical information related to this is described in non-patent literature 1 (“Tribology of Press Working”, edited by Japan Metal Stamping Association, written by Seiji Kataoka, issued by the Nikkan Kogyo Shinbun, Ltd.).
The prior art for forming micro pools on the surface of a metal member is disclosed by, for example, patent literature 1 (Japanese Patent Application Laid-Open No. 2005-144528), patent literature 2 (Japanese Patent Application Laid-Open No. 7-18403), and patent literature 3 (Japanese Patent Application Laid-Open No. 2001-247948).
Patent literature 1 discloses a mold for which lubricant pools with a size of approximately 3 μm×3 μm×1.5 μm are formed on a molding die surface of the mold surface that makes contact with the workpiece. When forming work is performed with a lubricant present between the mold and the workpiece, the lubricant is stored in the lubricant pools. Accordingly, the workpiece is formed with high lubricity kept between the mold die surface of the mold and the surface of the workpiece.
Patent literature 2 discloses a metal member in which micro pools with a depth of 2 μm or more, a density of 200 units/mm2 to 8200 units/mm2, and an area ratio of 10% to 70% are formed by heat treatment of alloy material.
Patent literature 3 discloses a metal member having grooves with a depth of 0.1 μm to 5.0 μm inclusive at an area ratio of 0.1% to 30.0% inclusive on the surface and particle projections with a diameter of 5 nm to 500 nm inclusive, viewed from the surface, a height of 5 nm to 500 nm inclusive at an area ratio of 30% or more (including 100%) in the surface other than the above.
The prior arts improve the holding characteristics of a lubricant by forming micro pools on the surface of the metal member. If this metal member is used repeatedly for processing, however, adhesion of the workpiece to the surface of the metal member occurs, thereby causing wear of the metal member and dimensional error of a molded product.